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Sluggish movement at altitude is partly a brain effect

By Nic Fleming

At high altitude, even the fittest mountaineer’s ability to move freely can vanish in the thin air. But it’s not the fault of your muscles. In fact, this drop-off in athletic performance in low-oxygen conditions may be mostly in the mind&colon; the brain kicks in to prevent potentially damaging overexertion.

The cause of muscle fatigue has been the subject of much debate. Some researchers emphasise the importance of physical changes such as lactic acid build-up, while others back a “central governor” theory whereby fatigue is a sensation generated by the brain.

Emma Ross of the University of Brighton, UK, and colleagues asked 11 men to carry out knee extensor muscle exercises while breathing normal air – which has 21 per cent oxygen – as well as mixes with 16 per cent, 13 per cent and 10 per cent oxygen to represent mild, moderate and severe hypoxia. As the oxygen levels fell, so did the forces the participants could generate voluntarily.

To assess the role of the brain in muscle fatigue, the team repeated the experiment using non-invasive brain stimulation to artificially generate motor cortex signals, overriding voluntary control and triggering knee muscle contraction.

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Measuring the difference between the forces participants could generate voluntarily and those created by the brain stimulation helped Ross and colleagues establish that the brain contributes 18 per cent to muscle fatigue with normal oxygen, 25 per cent for mild to moderate hypoxia and 54 per cent for severe hypoxia (Journal of Applied Physiology, DOI&colon; 10.1152/japplphysiol.00458.2010).

Ross and her colleagues then did similar tests on volunteers under normal oxygen conditions at sea level and low-oxygen conditions at the Pyramid Laboratory-Observatory on mount Everest, at an altitude of 5050 metres. The brain accounted for around 5 per cent of the reduction in force-generating capacity at sea level but 20 per cent at high altitude. The proportions were lower than in the first study because the exercises were shorter, says Ross.

As the participants acclimatised to high-altitude conditions, and the levels of oxygen in their blood increased, the brain’s role in muscle fatigue declined to around 13 per cent after seven days and 9 per cent after 14 days.

“Our findings provide the first direct evidence that when the human body is stressed beyond its normal limits, such as through exercise in a low-oxygen environment, the brain has an increased influence on our ability to use our muscles,” says Ross. “It could be an evolutionary mechanism to ensure we always have some capacity for movement.”

The research could help mountaineers plan their acclimatisation strategies. A better understanding of the causes of fatigue could also lead to improved treatments for people with low blood oxygen as a result of conditions such as chronic obstructive pulmonary disease and congestive heart failure.

The study “helps us to better understand the brain’s role in limiting exercise at altitude”, says Markus Amann of the University of Utah, Salt Lake City.